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Search: id:A054499
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| A054499 |
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Number of pairings on a bracelet; number of chord diagrams that can be turned over and having n chords. |
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+0
9
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| 1, 2, 5, 17, 79, 554, 5283, 65346, 966156, 16411700, 312700297, 6589356711, 152041845075, 3811786161002, 103171594789775, 2998419746654530, 93127358763431113, 3078376375601255821, 107905191542909828013, 3997887336845307589431
(list; graph; listen)
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OFFSET
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0,2
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COMMENT
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Place 2n points equally spaced on a circle. Draw lines to pair up all the points so that each point has exactly one partner. Allow turning over.
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LINKS
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A. Khruzin, Enumeration of chord diagrams
V. A. Liskovets, Some easily derivable sequences, J. Integer Sequences, 3 (2000), #00.2.2.
Index entries for sequences related to bracelets
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FORMULA
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a(n)=(2*A007769(n)+A047974(n)+A047974(n-1))/4.
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CROSSREFS
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Cf. A104256.
Adjacent sequences: A054496 A054497 A054498 this_sequence A054500 A054501 A054502
Sequence in context: A099825 A014288 A020096 this_sequence A001186 A125282 A020125
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KEYWORD
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nonn,easy,nice
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AUTHOR
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Christian G. Bower (bowerc(AT)usa.net), Apr 06 2000 based on a problem by Wouter Meeussen (wouter.meeussen(AT)pandora.be)
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EXTENSIONS
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Corrected and extended by N. J. A. Sloane (njas(AT)research.att.com), Oct 29 2006
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